In general, gasoline and other liquid hydrocarbon fuels boil in the range of about 100.degree. to about 650.degree. F. However, the crude oil from which these fuels are made contains a diverse mixture of hydrocarbons and other compounds which vary widely in molecular weight and therefore boil over a wide range. For example, crude oils are known in which 30 to 60% or more of the total volume of oil is composed of compounds boiling at temperatures above 650.degree. F. Among these are crudes in which about 10% to about 30% or more of the total volume consists of compounds so heavy in molecular weight that they boil above 1025.degree. F. or at least will not boil below 1025.degree. F. at atmospheric pressure.
Because these relatively abundant high boiling components of crude oil are unsuitable for inclusion in gasoline and other liquid hydrocarbon fuels, the petroleum refining industry has developed processes for cracking or breaking the molecules of the high molecular weight, high boiling compounds into smaller molecules which do boil over an appropriate boiling range. The cracking process which is most widely used for this purpose is known as fluid catalytic cracking (FCC). Although the FCC process has reached a highly advanced state, and many modified forms and variations have been developed, their unifying factor is that a vaporized hydrocarbon feedstock is caused to crack at an elevated temperature in contact with a cracking catalyst that is suspended in the feedstock vapors. Upon attainment of the desired degree of molecular weight and boiling point reduction the catalyst is separated from the desired products
Crude oil in the natural state contains a variety of materials which tend to have quite troublesome effects on FCC processes, and only a portion of these troublesome materials can be economically removed from the crude oil. Among these troublesome materials are coke precursors (such as asphaltenes, polynuclear aromatics, etc.), heavy metals (such as nickel, vanadium, iron, copper, etc.), lighter metals (such as sodium, potassium, etc.), sulfur, nitrogen and others. Certain of these, such as the lighter metals, can be economically removed by desalting operations, which are part of the normal procedure for pretreating crude oil for fluid catalytic cracking. Other materials, such as coke precursors, asphaltenes and the like, tend to break down into coke during the cracking operation, which coke deposits on the catalyst, impairing contact between the hydrocarbon feedstock and the catalyst, and generally reducing its potency or activity level. The heavy metals transfer almost quantitatively from the feedstock to the catalyst surface.
If the catalyst is reused again and again for processing additional feedstock, which is usually the case, the heavy metals can accumulate on the catalyst to the point that they unfavorably alter the composition of the catalyst and/or the nature of its effect upon the feedstock. For example, vanadium tends to form fluxes with certain components of commonly used FCC catalysts, lowering the melting point of portions of the catalyst particles sufficiently so that they begin to sinter and become ineffective cracking catalysts. Accumulations of vanadium and other heavy metals, especially nickel, also "poison" the catalyst. They tend in varying degrees to promote excessive dehydrogenation and aromatic condensation, resulting in excessive production of carbon and gases with consequent impairment of liquid fuel yield. An oil such as a crude or crude fraction or other oil that is particularly abundant in nickel and/or other metals exhibiting similar behavior, while containing relatively large quantities of coke precursors, is referred to herein as a carbo-metallic oil, and represents a particular challenge to the petroleum refiner.
In general, the coke-forming tendency or coke precursor content of an oil can be ascertained by determining the weight percent of carbon remaining after a sample of that oil has been pyrolyzed. The industry accepts this value as a measure of the extent to which a given oil tends to form non-catalytic coke when employed as feedstock in a catalytic cracker. Two established tests are recognized, the Conradson Carbon and Ramsbottom Carbon tests, the former being described in ASTM D189-76 and the latter being described in ASTM Test No. D524-76. In conventional FCC practice, Conradson carbon values on the order of about 0.05 to about 1.0 are regarded as indicative of acceptable feed.
Since the various heavy metals are not of equal catalyst poisoning activity, it is convenient to express the poisoning activity of an oil containing a given poisoning metal or metals in terms of the amount of a single metal which is estimated to have equivalent poisoning activity. Thus, the heavy metals content of an oil can be expressed by the following formula (patterned after that of W. L. Nelson in Oil and Gas Journal, page 143, Oct. 23, 1961) . in which the content of each metal present is expressed in parts per million of such metal, as metal, on a weight basis, based on the weight of feed: ##EQU1##
According to conventional FCC practice, the heavy metal content of feedstock for FCC processing is controlled at a relatively low level, e.g., about 0.25 ppm Nickel Equivalents or less.
The above formula can also be employed as a measure of the accumulation of heavy metals on cracking catalyst, except that the quantity of metal employed in the formula is based on the weight of catalyst (moisture free basis) instead of the weight of feed. In conventional FCC practice, in which a circulating inventory of catalyst is used again and again in the processing of fresh feed, with periodic or continuing minor addition and withdrawal of fresh and spent catalyst, the metal content of the catalyst is maintained at a level which may for example be in the range of about 200 to about 600 ppm Nickel Equivalents.
Petroleum refiners have been investigating means for processing reduced crudes, such as by visbreaking, solvent deasphalting, hydrotreating, hydrocracking, coking, Houdresid fixed bed cracking, H-Oil, and fluid catalytic cracking. Other approaches to the processing of reduced crude to form transportation and heating fuels named Reduced Crude Conversion (RCC) after a particularly common and useful carbo-metallic feed are disclosed in U.S. patent application Ser. Nos. 94,216, 94,217, 94,091, 94,227 and 94,092 all filed on Nov. 14, 1979, and which are incorporated herein by reference thereto. In carrying out the processes of these applications, a reduced crude is contacted with a hot regenerated catalyst in a short contact time riser cracking zone, and the catalyst and products are separated instantaneously by means of a vented riser to take advantage of the difference between the momentum of gases and catalyst particles. The catalyst is stripped, sent to a regenerator zone and the regenerated catalyst is recycled back to the riser to repeat the cycle. Due to the high Conradson carbon values of the feed, coke deposition on the catalyst is high and can be as high as 12 wt % based on feed. This high coke level can lead to excessive temperatures in the regenerator, at times in excess of 1400.degree. F. to as high as 1500.degree. F., which can lead to rapid deactivation of the catalyst through hydrothermal degradation of the active cracking component of the catalyst (crystalline aluminosilicate zeolites) and unit metallurgical failure.
As described in the above-mentioned co-pending reduced crude patent applications, excessive heat generated in the regenerator is overcome by heat management through utilization of a two-stage regenerator, regeneration of a high CO/CO.sub.2 ratio to take advantage of the lower heat of combustion of C to CO versus CO to CO.sub.2, low feed and air preheat temperatures and water addition in the riser as a catalyst coolant.
Various embodiments of regenerators and processes of regeneration useful in processing reduced crudes are described in the above-identified U.S. patent applications, including patent application Ser. Nos. 228,393, 246,751, 246,782, 258,265 and 290,277, and the material in these applications including that relating to regeneration of catalyst is hereby incorporated by reference.
As will be appreciated the carbo-metallic oils can vary widely in their Conradson carbon content. Such varying content of carbon residue in the feedstock, along with variations in riser operating conditions such as catalyst-to-oil ratio and others, can result in wide variations of the percent coke found on the spent catalyst.
In typical VGO operations employing a zeolite-containing catalyst in an FCC unit the amount of coke deposited on the catalyst averages about 4-5 wt % of feed. This coke production has been attributed to four different coking reactions, namely, contaminant coke (from metal deposits), catalytic coke (acid site cracking), entrained hydrocarbons (pore structure adsorption--poor stripping) and Conradson carbon. In the case of processing higher boiling fractions, e.g., reduced crudes, residual fractions, topped crude, etc., the coke production based on feed is the sum of the four kinds mentioned above including exceedingly high Conradson carbon values.
In addition, it has been proposed that two other types of coke-forming processes or mechanisms may be present in reduced crude processing in addition to the four exhibited by VGO. They are adsorbed and absorbed high boiling hydrocarbons not removed by normal efficient stripping due to their high boiling points, and carbon associated with high molecular weight nitrogen compounds adsorbed on the catalyst's acid sites.
This carbonaceous material is principally a carbonaceous, hydrogen-containing product as previously described plus high boiling adsorbed hydrocarbons with boiling points as high as 1500.degree.-1700.degree. F. that have a high hydrogen content, high boiling nitrogen containing hydrocarbons and porphorines-asphaltenes.
Coke production when processing reduced crude is normally and most generally about 4-5% plus the Conradson carbon value of the feedstock. As the Conradson carbon value of the feedstock increases, coke production increases and this increased load will raise regeneration temperatures. However, at adiabatic conditions, a limit exists on the Conradson carbon value of the feed which can be tolerated at approximately about 8 even at these higher temperatures. Based on experience, this equates to about 12-13 wt % code on catalyst based on feed.
That portion of the carbo-metallic feed which is not vaporizable at the temperatures encountered in the reactor tends to deposit as a liquid on the surfaces of the catalyst particles and is carried with the catalyst to the subsequent stages of the process. Steam stripping of adsorbed and absorbed gaseous hydrocarbons from the catalyst before it is introduced into the regenerator reduces the amount of material burned and heat produced within the regenerator. However, the high-boiling liquid constituents on the catalyst are not removed to a significant extent by conventional stripping techniques, and they contribute a significant amount of heat load to the regenerator, especially where the amount of material in feed which does not boil below about 1025.degree. F. exceeds about 10%. Some feeds may contain as much as 20% or even as much as 40% or 60% of material which does not boil below about 1025.degree. F. These high concentrations of high boiling point materials not only can place a high heat load on the regenerator, but their potential value as a liquid fuel or source of chemicals is lost by burning them in a regenerator.